Electroplating from an organic electrolytic solution



United States Patent 3,131,134 EIECTROPLATING FROM AN ORGANIC ELECTROLYTIC SOLUTlON Carl Micillo, Brooklyn, N. assignor to Grumman Aircraft Engineering Corporation, Bethpage, N. a corporation of New York No Drawing. Filed Aug. 3, 1961, Ser. No. 128,960 11 Claims. (Cl. 204-14) This invention relates to the art of electrodeposition of metals and more particularly to baths, compositions, and processes using non-aqueous organic electrolytic solutions for electrodepositing metals. In certain of its more specific aspects it relates to the electrodeposition of cadmium.

Electrodeposition of cadmium on high-strength steels has become a problem of vital importance because of the increasing use of steels heat-treated above 260,000 p.s.i. ultimate tensile strength. Steels heat-treated in these ranges are markedly susceptible to hydrogen absorption during the electroplating process. The presence of hydrogen in the steel reduces the fatigue strength of the metal through hydrogen embrittlement.

Electroplating with cadmium is customarily done from an aqueous cyanide bath. While this conventional process has favorable qualities, it is a known source for the introduction of hydrogen into metals such as steels. Modifications of the cyanide bath process have been successful in reducing the introduction of hydrogen into the metal, but these processes require either a baking treatment after plating to drive out the hydrogen, or extremely careful control of the plating solution and its constituents. Studies conducted by investigators in the field indicate that the amount of hydrogen that will result in the embrittlement of high-strength steel is quite small, and the amount required to cause embrittlement decreases as the strength level of the steel increases. Inasmuch as highstrength plated parts will not show signs of embrittlement until they have failed under sustained loading, the margin of safety is critically .-.small. Therefore, conventional processes, even with the latest modifications, are not successful to the required degree for steels heat-treated to a strength range where their susceptibility to hydrogen embrittlement becomes extremely high.

It is, therefore, a primary object of the present invention to provide a new process for the electrodeposition of metals from a non-aqueous organic electrolytic bath.

It is also an important object of this invention to provide a process for electroplating with cadmium whereby a sound adherent cadmium plating is obtained without a co-deposition of hydrogen which would result in the consequent hydrogen embrittlement of the plated metal.

Another object is to provide a process for electroplating with cadmium in which the danger of hydrogen embrittlement is avoided without post-plating operations such as bake-relieving and the like being required.

Still another object of this invention is an electroplating process with which a sound, adhesive, corrosion-resistant cadmium plate is deposited with a cathode efliciency approaching 100%.

A further object is the provision of a process for the electrodeposition of cadmium and other metals using a stable, organic electrolytic solution having an optimum margin of safety, with an indefinite life if properly operated, and requiring a minimum of control.

ICC

Yet another object of this invention is to provide a process for electroplating metals in which the toxic effects offer no more hazards for humans than the conventional cyanide processes when customary safety precautions are observed.

Further objects and advantages of the invention will become apparent from the following description.

Electroplating baths suitable for use in the process of my invention comprise a non-aqueous organic solvent selected from the amide family of compounds, a soluble halide salt of the metal to be electrodeposited, and an appropriate complexing agent.

Dimethylformamide is preferably used as a solvent on the basis of its relatively-high dielectric constant, and its ability to dissolve the salts of the metal to be plated. With its relatively-high dielectric constant, dimethylformamide resembles water closely as a good electrolytic solvent, and being in essence a substituted ammonia, it exhibits a number of the desirable characteristics of its parent solvent. Similarly, other acyl derivatives of ammonia in addition to dimethylformamide, such as dimethylacetamide, exhibit favorable properties for use as electrolytic solvents in the process of my invention.

The selection of the metal salt was made on the basis of its solubility in the electrolytic solvent and from its freedom from secondary reactions. In the electrodeposition of cadmium, cadmium iodide preferably is used because of its solubility in dimethylformamide and because no oxidation of the iodide ion occurs in the electroplating process due to the compensation made for the reduction of cadmium at the cathode. Cadmium undergoes dependable corrosion at the positive electrode; therefore, its oxidation at the anode balances out the reduction of cadmium at the cathode in an equilibrium reaction. For this reason, a cadmium metal electrode is used for the anode when cadmium is to be electrodeposited. The same condition exists when depositing other metals such as Zinc or tin; the anode used must be made of the metal to be deposited.

A metal complexing structure into which the cadmium is bound is dictated by the high concentration of the metal salt and its mechanism of dissolution in the bath. In the concentrated bath of this invention, the presence of the complexed cadmium salt insures a solution rich in dissolved metal salt and few metal ions, a condition comparable to the one found in conventional cadmium cyanide baths. The complexing agent is selected from a class of amines (i.e., any of a class of organic compounds of nitrogen considered to be derived from ammonia (NH by replacing one or more of the hydrogen atoms by organic radicals, such as CH or C l-I and is preferably an aliphatic, high-molecular weight derivative of ethylene diamine. The concentration of the complexing agent is determined by the coordination number of cadmium. This number is a guide that determines the number of molecules or ions of the complexing agent that will be bound to a single metal ion. Cadmium has a coordination number of four and will combine with four electron donor groups. Since the complexing agent preferably chosen here is a bidentate, containing two donor groups, only two molecules are required to bind one cadmium ion. To insure a complete binding, a molar concentration of complexing agent to cadmium of 2 to 1 is required. This ratio may vary when metals other than cadmium are elcetrodeposited, as the ratio depends on the nature 3 and coordinating number of the metal to be plated and upon the complexing agent employed. The correct ratio must be employed to ensure that all ions of the plating metal are coordinated prior to deposition.

During the plating operation, cadmium ions become reduced to cadmium metal when an electric current is passed through the solution. As a soluble cadmium anode is used, the reaction at the positive electrode may involve either the oxidation of cadmium metal to cadmium ions, or the oxidation of iodide ions to iodine. These reactions can be stated as follows:

I. Cd 2e- =Cd++ (at the anode) Cd ++2e =Cd (at the cathode) Od Cd Cd ++Cd E.m.f.= (or equilibrium) II. 21* 2e =Iz (at the anode) Cd -2e- =Cd (at the cathode) The first reaction, being at equilibrium, will always occur in preference to the second reaction. That the first reaction which involves the oxidation and reduction of the cadmium metal is the only reaction occurring when a soluble cadmium anode is employed has been substantiated both by visual checks and by amperometric titration.

A requirement of paramount importance in the successful operation of an electroplating process is an optimum current efliciency of the electrolysis in the plating bath. In the electrodeposition of cadmium, the quantity of metal deposited at the cathode or liberated at the anode should be proportional both to the quantity of electricity that passes through the solution and to the equivalent weight of the metal itself. The ratio of metal actually deposited by a given current to the maximum weight of that metal which could be deposited according to Faradays law is known as cathode efficiency. As tested in the laboratory, the process of the present invention exhibits a cathode efficiency closely approaching 100%.

A slightly higher anode area than cathode area is preferred in this invention to compensate for the solution lost by drag out. An approximate anode-cathode ratio of 1.5 to 1 sufiices to keep the cadmium metal content at an acceptably constant level.

The electroplating process of my invention can be carried out in the following manner using a cadmium iodidedimethylformamide bath composed of 70 to 130 grams of cadmium iodide per liter of solution (22 to 42 grams cadmium per liter of solution by analysis), ethylene diamine in a molar ratio of 2 to 1 of ethylene diamine to cadmium. Other halide salts of cadmium may be used provided the cadmium ion concentrations of the solution correspond to that obtained with 70 to 130 grams of cadmium iodide per liter of solution.

The vessel for the plating bath may be of any suitable design, and may be preferably made of glass, ceramic, chemical stoneware, or any suitable non-conducting material. The process is best conducted at a temperature within the approximate range of 70 F. to 90 F., with the maximum temperature permited being 130 F. Relative humidity may range up to a maximum of 50%, but 25% is preferred. As stated previously, cadmium anodes are used, and the preferred anode-cathode area ratio is 1.5 to 1. With a cathode-bar agitation of 180 feet per hour maximum, the cathode efiiciency closely approaches 100%. The current supplied to the cathode may be of as high a voltage as required to give a current density for plating of from about four to ten amperes per square foot of cathode area. Filtration may be used as required.

In the plating process of my invention, the first step is that of determining the surface area of the parts to be plated. Then the parts are sandblasted to clean and abrade the substrate. The parts are initially cleaned by sandblasting rather than by pickling to eliminate the hydrogen embrittlement known to be caused by pickling alone. The parts are then immersion dipped in a suitable caustic solution cleaner at l60-180 F. for five minutes, or ultrasonic dipped for one minute. The cleaner is removed by a water rinse and the parts are blown dry immediately. This is followed with an acetone dip, or toluene vapor degreasing may be used. The parts are then connected to the cathode bar of the plating vessel, judicious contact points at low current-density areas being made whenever possible. Cathode bar agitation is begun and plating is done at from 4 to 10 amps/ft. until .0003.0005 inch cadmium plating is deposited. The plating rate ranges from approximately .0004 in./hr. at 4 amp/ft. to approximately .00095 in./hr. at 10 amp./ ft. with a metallic electroplate containing 99.9% cadmium (by analysis) being deposited. Plating is followed by a thorough pressure rinse of the plated parts to ensure the complete removal of any iodide from the surfaces. After this rinse, a chromate conversion coating is applied.

Maintenance of the cadmium metal within concentration limits is the only control requirement of the process of my invention. This control is not critical because the limits of concentration are wide and because the hydrogen embrittlement free characteristics of the plating solution are not dependent upon metal content. The concen-v tration of complexing agent is kept constant to compensate for drag-out losses by the addition as required of a replenishing solution of ethylene diamine and dimethylformamide made up proportional to their original concentrations.

The metal content of the solution may be analyzed by means of titration with ethylenediamine-tetra-acetic acid..

In this process, low cadmium concentrations causes spongy or burnt deposits; too high a cadmium concentration requires more complexing agent and thus increases the viscosity of the bath. This increased viscosity requires that a higher speed of agitation be used and results in a reduction in cathode efliciency.

Sound cadmium deposits are obtainable by this process in a relatively-wide current density range of between 3 and 12 amps./ft. At the low-current density range, deposition will be slow and the anodes will dissolve with relatively heavy smuts. In the big -current density range, slight burning on the edges will appear. This can be eliminated by increased agitation at the expense of cathode efiiciency.

A degree of hygroscopicity is exhibited by the nonaqueous solvent employed. The amount of water pick-up is a function of the relative humidity and mode of exposure. Laboratory tests have shown that the maximum water gain for dimethylformamide at 25 relative humidity is 14%. Since test specimens plated in solutions that were contaminated with up to 50% water showed no deleterious hydrogen embrittlement, no embrittlement failures can take place if the relative humidity (near the plating vessel) is kept at or about 25%. However, should the plating solution pick up an unacceptable amount of water contamination, the solution must be discarded.

Adhesion and corrosion protection characteristics were tested in accordance with Federal Specification QQ-P- 4l6a, Plating, Cadmium (Electrodeposited). Bend tests were performed using machines employed to check production parts. Flat plated specimens were bent back on themselves. Corrosion resistance was tested in a standard salt-spray cabinet. In the corrosion resistance tests a number of the cadmium plated specimens had a subsequent chromate conversion coating as it is anticipated that most production parts would be so protected. When tested in salt spray, these specimens showed no indication of white corrosion. Other specimens without a chromate conversion coating showed no evidence of red corrosion after being subjected to the salt spray test. The specimens with which this series of tests were conducted performed satisfactorily and compared favorably with tests of deposits obtained from a conventional cadmium cyanide bath.

Determination of delayed failure due to hydrogen embrittlement was made by the sustained-load. test method employing notched tensile bars of high-strength steels. Tests were conducted at 75% and 90% of the notched breaking strength for a minimum of 200 hours. As no specimens failed within 200 hours, even specimens plated with up to 50% water present in the plating solution, it is assumed that no embrittlement had taken place.

Since the principle of the invention herein described may be variously practiced without departing from the spirit or scope of the invention, it is to be understood that specific embodiments of the invention appearing in the above description are to be taken as illustrative and not in limitation except as may be required by the appended claims.

Having thus described my invention, what I claim is:

1. The process of electrodepositing a plating metal on steel and other metallic members in which a soluble anode of the plating metal is used as an anode and the metallic member is used as a cathode, which comprises immersing said metallic member in an electrolytic solution maintained at a temperature below approximately 130 F. and comprising a non-aqueous organic solvent selected from the amide group consisting of dimethylformamide and dimethylacetamide, a salt of the plating metal soluble in said solvent, and a derivative of ethylene diamine in amount sufiicient to complex substantially all of the metal ions in the solution, and passing a plating current through said solution from said anode to said metallic member.

2. The method of claim 1 wherein said salt of the plating metal is an halide.

3. The method of claim 1 wherein said organic solvent is dimethylformamide.

4. The method of claim 1 wherein said organic solvent is dimethylacetamide.

5. The method of claim 1 wherein said ethylene diamine derivative is an aliphatic, high-molecular weight derivative of ethylene diamine.

6. The process of electrodepositing cadmium on steel and other metallic members in which a soluble cadmium anode is used as an anode and said metallic member is used as a cathode, which comprises immersing said metallic member in an electrolytic solution maintained at a temperature within the approximate range of F. to F. and comprising the non-aqueous organic solvent dimethylformamide, cadmium ions in concentrations corresponding to that obtained in a solution containing 70 to grams of cadmium iodide per liter of solution and ethylene diamine in an amount snflicient to complex substantially all of the metal ions in the solution, and passing a plating current through said solution from said cadmium anode to said metallic member.

7. The method of claim 6 wherein the ethylene diamine and cadmium are in a molar ratio of about 2 to l.

8. The method of claim 1 wherein said electrolytic solution is maintained at a temperature within the approximate range of 70 F. to 90 F.

9. The method of claim 1 wherein the plating current density ranges from approximately 3 amperes per square foot to approximately 12 amperes per square foot.

10. The method of claim 1 wherein cathode bar agitation is maintained at a rate below approximately feet per hour.

11. The process of electrodepositing cadmium on steel and other metallic members in which a soluble cadmium anode is used as an anode and said metallic member is used as a cathode, which comprises immersing said metallic member in an electrolytic solution maintained at a temperature below approximately 130 F. and comprising a non-aqueous organic solvent selected from the amide group consisting of dimethylformamide and dimethylacetamide, cadmium ions in concentrations corresponding to that obtained in a solution containing 70 to 130 grams of cadmium iodide per liter of solution, and a derivative of ethylene diamine in an amount sufiicient to complex substantially all of the cadmium ions in the solution, and passing a plating current through said solution from said cadmium anode to said metallic member.

Certa et al. Dec. 31, 1957 Strauss et al. May 31, 1960 

1. THE PROCESS OF ELECTRODEPOSITING A PLATING METAL ON STEEL AND OTHER METALLIC MEMBERS IN WHICH A SOLUBLE ANODE OF THE PLATING MEAL IS USED AS AN ANODE AND THE METALLIC MEMBER IS USED AS A CATHODE, WHICH COMPRISES IMMERSING SAID METALLIC MEMBER IN AN ELECTROLYTIC SOLUTION MAINTAINED AT A TEMPERATURE BELOW APPOXIMATELY 130*F. AND COMPRISING A NON-AQUEOUS ORGANIC SOLVENT SELECTED FROM THE AMIDE GROUP CONSISTING OF DIMETHYLFORMAMIDE AND DIMETHYLACETAMIDE, A SALT OF THE PLATING METAL SOLUBLE IN SAID SOLVENT, AND A DERIVATIVE OF ETHYLENE DIAMINE IN AMOUNT SUFFICIENT TO COMPLEX SUBSTANTIALLY ALL OF THE METAL IONS IN THE SOLUTION, AND PASSING A PLATING CURRENT THROUGH SAID SOLUTION FROM SAID ANODE TO SAID METALLIC MEMBER. 